You could try going to a flooring/ceramic tile store and ask for some
scraps of granite, slate, marble, and limestone tile.
Use a 3" diamond hole saw to make disks that you could set at various
levels in your concrete cores?

"Gerry" wrote in message
...
I have a project for LSU to build a reduced size oil drilling rig for
classroom use. Rig is made and has instrumentation to mimic the real
thing. Next phase is to make cores for it to drill through, simulating
different type and layers of formation. Been playing with parrifin and
cement combination but am looking for other ideas, like maybe some
epoxy/aggragate that will offer different drilling rates. Bit is a 1"
masonry bit, turning at speeds below 150 RPM. Cores are made up in 3"
PVC pipe with a 1/8 hole cast through the center to provide a flow
path for water to carry cuttings from hole. Any ideas for core
materials will be appreciated. Thus far I have used cement/sand with
varying levels of cement, and wax with bentonnite or sand. Both work
but I need more hardness to vary drilling rates